1. Field of the Invention
The invention relates to telecommunications. More particularly, the invention relates to methods and apparatus for calculating a pointer leak rate for retiming a SONET signal.
2. State of the Art
Since the early nineteen sixties, three different digital multiplexing and signaling hierarchies have evolved throughout the world. The hierarchies were developed in Europe, Japan, and North America. Fortunately, all are based on the same pulse code modulation (PCM) signaling rate of 8,000 samples per second, yielding 125 microsecond sampling slots (1 second/8,000 samples=0.000125). Japan and North America base their multiplexing hierarchies on the DS-1 rate of 1.544 Mbit/sec±20 ppm, although the higher data rates in Japan do not correspond to the higher rates used in North America. Europe bases multiplexing on a rate of 2.048 Mbits/sec called E1 which carries thirty voice circuits compared to the twenty-four carried in the DS-1 rate. The next most common higher rates in the U.S. and Europe are DS-3 and E3, respectively, which have rates of 44.736 Mbit/sec±20 ppm and 34.368 Mbit/sec±20 ppm, respectively.
The Synchronous Optical Network (SONET) or the Synchronous Digital Hierarchy (SDH), as it is known in Europe, is a common transport scheme which is designed to accommodate both DS-1 and E1 traffic as well as multiples (DS-3 and E3) thereof. Developed in the early 1980s, SONET has a base (STS-1) rate of 51.84 Mbit/sec in North America. In Europe, the base (STM-1) rate is 155.520 Mbit/sec, equivalent to the North American STS-3 rate (3*51.84=155.520). The abbreviation STS stands for Synchronous Transport Signal and the abbreviation STM stands for Synchronous Transport Module. STS-n signals are also referred to as Optical Carrier (OC-n) signals when transported optically rather than electrically.
The basic STS-1 signal which has a frame length of 125 microseconds (8,000 frames per second) and is organized as a frame of 810 octets (9 rows by 90 byte-wide columns). The first three columns of each row consist of transport overhead (TOH). Of these twenty-seven octets, nine are allocated for section overhead and eighteen are allocated for line overhead. The remainder of the frame (9 rows of 87 columns=783 octets) is referred to as the envelope or Synchronous Payload Envelope (SPE) or, in Europe, the Virtual Container. The first column of the envelope is reserved for STS path overhead (POH) and is referred to as the transport part of the envelope. The remaining 86 columns is referred to as the user part of the envelope. “Path” represents the complete transit through the SONET network. “Line” represents transit from one multiplexer to another. “Section” represents transit from one network element to another.
In order for data to be accommodated efficiently in the SPE, the 87 bytes of the SPE are divided into three blocks each including 29 columns. The POH occupies column 1 and “fixed stuff” (bytes which convey no information) is inserted into the 30th and 59th columns. Data is accommodated in the remaining 3*28=84 columns=756 bytes. An STS-n signal is comprised of n STS-1 signals which are frame aligned and byte-interleaved. An STS-nC signal is comprised of n STS-1 signals which are frame aligned and concatenated. Currently, the highest level STS signal is STS-192 which has a line rate of 9,953.28 Mbit/sec.
These various synchronous optical network signals contain payload pointers which provide a method of allowing flexible and dynamic alignment of the SPE (Virtual Container) within the envelope or container capacity, independent of the actual contents of the envelope or container. Dynamic alignment means that the STS or STM respective SPE or Virtual Container is allowed to float within the STS/Virtual Container envelope capacity/container. For example, an STS-1 SPE may begin anywhere in the STS-1 envelope capacity. Typically, it will begin in one STS-1 frame and end in the next frame. The STS payload pointer is contained in the H1 and H2 bytes (the first two bytes) of the line overhead. These two bytes designate the location of the payload byte (the J1 byte) where the STS SPE begins.
When first generated, an SPE is aligned with the line overhead at the originating node (i.e., the pointer value is fixed to some value from 0 to 782). As the frame is carried through a network, however, it arrives at intermediate nodes (e.g., multiplexers or cross-connects) having an arbitrary phase with respect to the outgoing transport framing of the intermediate nodes. If the SPE had to be frame-aligned with the outgoing signal, the frame would need to be buffered and delayed. Thus, the avoidance of frame alignment allows SPEs on incoming links to be immediately relayed to outgoing links without artificial delay. The location of the SPE in the outgoing payload envelope is specified by setting the H1, H2 pointer to the proper value (0-782). The pointer values are regenerated at each intermediate node in the network.
In addition, if there is a frequency offset between the frame rate of the transport overhead and that of the STS SPE, then the pointer value will be incremented or decremented, as needed, accompanied by a corresponding positive or negative stuff byte. If the frame rate of the STS SPE is too slow with respect to the transport overhead, then the alignment of the envelope must periodically slip back in time, and the pointer must be incremented by one. This operation is indicated by inverting selected odd bits (I-bits) of the pointer word to allow five-bit majority voting (or 8 out of 10 as per Bellcore requirements) at the receiver. A positive stuff byte appears immediately after the H3 byte in the frame containing inverted I-bits. Subsequent pointers will contain the new offset value.
If the frame rate of the STS SPE is too fast with respect to that of the transport overhead, then the alignment of the envelope must be periodically advanced in time, and the pointer must be decremented by 1. This operation is indicated by inverting selected even bits (D-bits) of the pointer word to allow five-bit majority voting at the receiver. A negative stuff byte appears in the H3 byte in the frame containing the inverted D-bits. Subsequent pointers will contain the new offset value.
Previously incorporated U.S. Pat. No. 5,331,641 discloses methods and apparatus for retiming and realignment of STS-1 signals into an STS-3 type signal. The SPE of an incoming STS-3 type signal is demultiplexed into three STS-1 payloads and fed to three FIFOs, and a byte which is synchronous with the TOH is tracked through the three FIFOs to provide an indication of the FIFO depth. A frame count is also kept to track the number of frames since a last pointer movement. Stuffs or destuffs are generated based on the FIFO depth as well as based on the frame count, with a stuff or destuff generated as quickly as four frames from a previous pointer movement if the FIFO is close to full or close to empty, and less quickly (e.g., at thirty-two frames from a previous pointer movement) if the FIFO is only starting to empty or to fill. The '641 patent mentions a “pointer recalculation block” but does not disclose details about pointer recalculation.
Pointer recalculation is generally accomplished by summing the positive and negative pointer movements with the previous pointer. There are practical limits to the frequency and magnitude of pointer movements and these are part of the SONET standards. In order to assure that pointer movements do not exceed the number permitted by the standards and also in order to avoid unnecessary pointer movements, it is known to “leak” pointer movements. Pointer leaking involves buffering several pointer movements, summing them, and leaking the net pointer movement after a period of accumulating pointer movements. The '641 patent does not teach how to leak pointers. Advantageously, the pointer leak rate is adjustable based on the amount of jitter in the STS signal. Known methods of calculating a pointer leak rate are often cumbersome in terms of the amount of code needed to calculate the leak rate.
Previously incorporated U.S. Pat. No. 6,577,651 discloses a method for retiming and realigning SONET signals and includes demultiplexing STS-1 signals from an STS-3 signal, buffering each of the three signals in a FIFO, determining the FIFO depth over time, and determining a pointer leak rate based in part on FIFO depth and also based on the rate of received pointer movements. The apparatus of the '651 patent includes a dual port RAM FIFO, a write address generator based on the incoming signal clock, a read address generator based on the outgoing signal clock, a FIFO depth counter synchronized to a First_Byte of the SPE written to the FIFO, a FIFO depth latch synchronized to the First_Byte read from the FIFO, and circuitry for calculating leak rate based on FIFO depth and number of pointer movements received over a period of time.
According to the disclosed preferred embodiment, each FIFO is 28 bytes deep. If the depth of a FIFO is 12-16 bytes, no pointer leaking is performed. If the depth is 0-4 bytes, an immediate positive leak is performed. If the depth is 24-28 bytes, an immediate negative leak is performed. If the depth is 5-11 bytes, a calculated positive leak is performed. If the depth is 17-23 bytes, a calculated negative leak is performed. The calculated leak rates are based on the net number of pointer movements (magnitude of positive and negative movements summed) received every 32 seconds (256,000 frames). A look up table is provided with nine leak rates, expressed as a number of milliseconds between leaks. If the number of pointer movements received in the last 32 seconds is less than 32, the leak spacing is 960 ms. The relationship between leak spacing values and the number of pointer movements received in the last 32 seconds is linear. I.e., if the pointer movement rate is doubled, the leak spacing time is halved. The calculated leak rate is preferably reset to minimum spacing every time a new pointer occurs and every time a loss of frame occurs.
Although they are efficient and accurate, the solutions disclosed in the '651 patent have three disadvantages. First, the number of leaked pointers is always slightly greater than the asking rate (i.e. the net number of justifications) because an exact match of the rates cannot be achieved and a lower number would result in buffer underflow/overflow in situations of long-term steady pointer movements. In other words, the system of the '651 patent allows for errors on the safe side by overleaking pointers. While this is not a problem on its own, it can be a problem in a system of retimers each of which allow for this kind of error. As the signal travels through the system, leak rate error can be multiplied resulting in buffer underflow/overflow down the line. Second, since the averaging process is repeated every 32 seconds, there is a possibility of bursty leak rates. Third, there is relatively little control in delaying outgoing pointer justifications for properly sequencing cascaded retimers.